I. Field of the Invention
The present invention relates generally to the field of detecting the commutation position of a rotor within a polyphase brushless motor. More particularly, the present invention is directed to an apparatus and method capable of providing real-time commutation position detection by continuously differentiating the electric current flowing within the active stator coils of a polyphase brushless motor.
II. Discussion of the Related Art
Generally speaking, brushless motors include a rotor and a stator having a plurality of wound field coils. Brushless motors have gained increasing popularity and enjoy a wide array of industrial applications due, in large part, to the fact that brushless motors are electronically commutated, wherein solid-state switching replaces the brushes and segmented commutators of traditional permanent magnet DC motors. The elimination of brushes simplifies motor maintenance as there are no brushes to be serviced or replaced. Furthermore, noise reduction is effectuated because, without brushes, there is no arcing to create electromagnetic interference. The elimination of arcing also minimizes any explosion hazard in the presence of flammable or explosive mixtures. Thus, brushless motors are ideal for use in any setting where sensitive circuitry or hazardous conditions exist or are present.
Brushless motors may be of the variable reluctance, permanent magnet, or hybrid type. Variable reluctance brushless motors are characterized by having an iron core rotor chase sequentially shifting magnetic fields of the stator coils to attract the rotor into rotational motion. Permanent magnet brushless motors are characterized by having the sequentially energized field coils attract or repel a permanent magnet rotor into rotational motion. Hybrid brushless motors, such as stepper motors, are operated by a train of pulses so that their rotors move or are indexed over a carefully controlled fraction of a revolution each time they receive an input step pulse. This permits rotor movement to be controlled with high precision which can be translated into precise rotational or linear movement.
To ensure proper rotational and linear movement in variable reluctance and permanent magnet brushless motors, it is essential to determine the position of the rotor with respect to the energized, or active, stator coils. By knowing this position, referred to as commutation position, the stator coils can be energized in the appropriate sequence to create a revolving magnetic field in the motor to exert the desired rotational or linear torque on the rotor. Traditionally, commutation position is detected by employing one or more transducers within the particular brushless motor to sense the position of the rotor relative to the active stator coil or coils.
However, the use of transducers to determine commutation position has several drawbacks. First, these sensors increase production costs due to the need for sophisticated positional adjustment and increased wiring. Moreover, the space required for commutation position sensors is also a significant disadvantage in that valuable space is consumed within the motor housing. With an ever increasing premium on space and cost efficiency, several attempts have been made to create "sensorless" commutation position feedback systems to replace the need for commutation position sensors within brushless motors.
U.S. Pat. No. 5,327,053 to Mann et al. employs one such "sensorless" technique, wherein the back-EMF voltage in an unenergized stator coil is employed to determine commutation position during motor start-up. This method is based on a proportional relationship, wherein the back-EMF voltage generated in the stator coils during motor operation is a function of motor speed, rotational direction, and commutation position. However, a significant disadvantage exists with this technique in that back-EMF voltage is difficult to reliably measure during the low rotational velocity of the rotor during start up operations. This may prove especially troublesome in applications such as computer disk drive motors, where proper rotational direction at start up is required to avoid damaging disk contents.
U.S. Pat. No. 5,191,270, issued to McCormack represents an attempt to overcome the disadvantages of the back-EMF method. In this technique, "sensorless" commutation detection is performed during the start up phase of operation. An initial measurement is made of the current rise time within each stator coil by applying a known voltage to each stator coil while the rotor is held stationary. A driving current is then supplied to the stator coil which is most nearly aligned with the magnetic field of the rotor so as to move the rotor slightly. A second current rise time measurement is conducted in similar fashion and compared to the initial current rise time measurement. Measurements of initial and second current rise times are analyzed for each stator coil to indicate which stator coil should be energized first to provide proper rotational direction of the rotor at start-up.
A major disadvantage exists, however, in that this method is limited solely to start-up operations. In particular, this method is aimed at determining the rotational direction of a computer disk drive during start-up so as to avoid damaging disk contents from improper rotational direction. To accomplish this, the current rise time within each stator coil is measured at two discrete intervals merely to determine which of the stator field coils should be initially energized to start the rotor in the correct rotational direction. This method, however, does not account for the commutation position of the rotor with respect to the stator field coils continuously throughout the normal, full speed operation of the motor.
Furthermore, this method does not provide an accurate assessment of commutation position. As mentioned above, this method merely measures the current rise times in all stator coils at two discrete points in time, compares these values, and initiates rotation accordingly. By basing the commutation position detection on two discrete measurements, this method must assume certain positional characteristics that cannot be assessed during the interim between current rise time measurements. These assumptions cause this method to be accurate only within one commutation, as opposed to the entire operational cycle. This can be a significant hindrance to proper motor operation because the stator coils cannot be continuously driven in an efficient fashion without an accurate and continuous determination of commutation position.
A further disadvantage of this method is that the determination of commutation position is based upon the absolute value of the current flowing within the motor. By measuring the current rise times within each stator coil at fixed time intervals, this method is highly susceptible to variations or fluctuations in motor speed, motor load, PWM frequency, and bus voltage. For example, an increase in the motor load will require the pulse width of the driving signal to be lengthened to increase the energy supplied to the motor to compensate for the increased load. Such variations in pulse width cause the current rise time measurement to vary in amplitude depending on the load experienced, thereby adversely affecting the accuracy and reliability of the commutation position detection.
Still another drawback of this method is that it requires a substantial amount of processing time to determine commutation position. This results from the need to measure current rise time twice for each stator coil, as well as the subsequent comparisons of the current rise time measurements for each stator coil. For example, a four phase brushless motor employing this method requires eight current rise time measurements, one for each stator coil prior to moving the rotor and one for each stator coil after the rotor has been moved. Each pair of current rise time measurements must then be compared to indicate the relative change in current rise time within each stator coil. Finally, the relative change of current rise time within each stator coil must be compared to indicate which stator coil is positioned closest to the rotor so that the particular stator coil can be commutated to drive the rotor into rotation.
What is needed therefore, is a commutation position detection system which is capable of accurately and continuously assessing commutation position throughout the entire commutation process, rather than merely at start-up. A need also exists for a commutation position detection system that is not dependent on the absolute value of the current flowing within the motor. Still another need exists for a commutation position detection system that is fast and requires minimal processing time.